An MRI apparatus is a medical-use diagnostic noninvasive imaging apparatus, utilizing nuclear magnetic resonance phenomenon. The nuclear magnetic resonance phenomenon indicates that hydrogen nucleus (protons) placed in a static magnetic field, are resonant with an RF magnetic field at a specific frequency. Since nuclear magnetic resonance signals are changed depending on various physical properties, such as proton density and relaxation time, an image obtained by the MRI can depict various biological information items, such as a structure or a composition of living tissue and cell properties.
In recent years, a magnetic susceptibility difference between living tissues receives attention, as one of the physical properties being measurable by the MRI. The magnetic susceptibility is a physical property that represents a degree of magnetic polarization (magnetization) of materials in the static magnetic field. In a living body, there are contained paramagnetic substances such as deoxyhemoglobin and iron protein in venous blood, and diamagnetic substances such as water constituting a large part of the living tissue and calcium serving as a basis of calcification. Creating an image quantitatively from a magnetic susceptibility difference between the living tissues, or the magnetic susceptibility difference being weighted, may be applicable to diagnosis of cerebral ischemia disease, prediction of radiation treatment effect against cancer, and identification of neurodegenerative disease.
A method for creating an image of the magnetic susceptibility difference between living tissues by utilizing the MRI is referred to as quantitative susceptibility mapping (QSM). A method for creating an image by weighting the magnetic susceptibility difference between living tissues is referred to as susceptibility weighted imaging (SWI). The QSM is a method of calculating local magnetic field changes caused by the magnetic susceptibility difference between living tissues, from phase information of an MR image being measured, and obtaining a quantitative susceptibility distribution according to a relational expression between the magnetic field and the magnetic susceptibility. The SWI is a method of calculating a weighting mask image where local magnetic field changes are weighted, and multiplying a measured weighted image (absolute value image) by thus calculated weighting mask, thereby obtaining an image where the magnetic susceptibility is weighted.
In order to obtain the quantitative susceptibility distribution or the susceptibility weighted image, according to the QSM or the SWI, it is necessary to calculate the local magnetic field changes that are caused by the magnetic susceptibility difference between living tissues. Usually, by using the Gradient echo (GrE) method, a distribution of the local magnetic field changes (local magnetic field distribution) is calculated from a phase distribution that is measured at one echo time (TE). Specifically, the phase distribution is calculated from a measured complex image, a phase aliasing removal process is performed for removing phase aliasing that occurs within the range from −n to +n in thus calculated phase distribution. Thereafter, a background removal process is performed for removing global magnetic field (background magnetic field) changes caused by a subject shape or other factors, thereby obtaining the local magnetic field that is caused by the magnetic susceptibility difference between living tissues.
An image quality of the local magnetic field distribution depends on a signal-to-noise ratio (SNR: Signal Noise Ratio) of the measured phase distribution. It is known that in the phase distribution of MRI, the SNR of the phase distribution is maximized when measurement is performed at the TE corresponding to an apparent transverse magnetization relaxation time T2* of the living tissue being a target. On the other hand, in the phase aliasing removal process, it is not possible to increase the length of TE, so as to remove the phase aliased over the range from −n to +n without fail. Accordingly, a phase distribution with an optimum SNR cannot be acquired if the TE is only one. In view of this, multi-echo measurement for acquiring images at a plurality of TEs allows obtaining the local magnetic field distribution with an optimum SNR, without failing in removing the phase aliasing.
There are suggested several methods for calculating the local magnetic field distribution from the phase distributions at a plurality of TEs obtained by the multi-echo measurement.
As representative methods, there are two methods as the following.
In one of the two methods (referred to as conventional method 1), in the phase distribution of the multi-echo images measured at a plurality of TEs, a phase aliasing removal process in the time direction is performed. Then, linear fitting is applied using a feature that the phase in the time direction varies linearly, thereby calculating a frequency component caused by static magnetic field inhomogeneity. Next, spatial frequency aliasing removal process is performed, followed by the background removal process, and a local magnetic field distribution is obtained (e.g., see Non Patent Document 1).
In the other method (referred to as conventional method 2), in the phase distribution of the multi-echo images measured at a plurality of TEs, the phase aliasing removal process and the background removal process are performed for the phase distribution of each echo individually, thereby obtaining the local magnetic field distribution of each echo. Then, weighted averaging is applied to the local magnetic field distributions of the respective echoes, so as to obtain a final local magnetic field distribution (e.g., see Patent Document 1).